The present invention relates to furnaces and kilns for heat treating materials at relatively high temperatures and to combination thermal insulating and heating units which are used in forming the enclosure for such furnaces and kilns, hereafter referred to as "furnaces."
The present invention is an improvement upon ovens utilizing the thermal insulating and heating unit of the present inventor's U.S. Pat. No. 4,575,619 entitled ELECTRICAL HEATING UNIT WITH SERPENTINE HEATING ELEMENT granted on Mar. 11, 1986, and an improvement on such thermal insulating and heating units. That patent describes a combination thermal insulating and heating unit in which a serpentine heating element is disposed on the floor of a slot which extends into a block of ceramic fibers. The patent also describes the process of making such a combination thermal insulating and heating unit of ceramic fibers in which a serpentine heating element is supported on a narrow elongated strip which rests on a liquid permeable screen at the bottom of a vacuum box, the heating element having reverse bends protruding from both sides of the elongated strip. A slurry of ceramic fibers, water and a binder is then poured into the vacuum box covering the serpentine heating element to a satisfactory depth. A portion of the water and binder of the slurry is then permitted to drain from the vacuum box, and thereafter a vacuum is drawn below the screen to cause more of the water and binder of the slurry to drain through the screen. The vacuum also causes the fibers to pack tightly forming a mat with sufficient integrity to permit removal from the vacuum box. The thin strip is then removed exposing the central portions of the heating element on the bottom or floor of an elongated slot formed by the supporting strip for the heating element. The mat is then cured in an oven to form a combination thermal insulation and heating element in which the element is disposed at the bottom or floor of a slot.
One of the advantages of the structure described above, is that the serpentine heating element is securely mounted on the block of ceramic fibers by outwardly extending bends which are embedded in the ceramic fiber block, but a portion of the heating element is directly exposed to the interior of the enclosure by the slot to provide efficient heat transfer. While this construction has proven to provide a superior mounting construction for the heating element on the ceramic fiber mat and good heat transfer, it places restrictions on the heating element itself which have made it difficult to increase the heat transferred from a given area of the combination thermal insulating and heating unit. As a result of these restrictions, efforts to shorten the time required to bring a furnace constructed with such thermal insulating and heating units to operating temperature have been frustrated. It is a primary object of the present invention to provide a furnace utilizing combination insulating and heating units with electrical heating elements anchored in a ceramic fiber block in which the furnace can be brought to operating temperature in a significantly shorter time than prior furnaces.
The heat produced by an electrical resistance element is determined by the formula: EQU P=I.sup.2 R;
where P is the power in watts, I is the electrical current through the resistance element in amperes, and R is the resistance in ohms. Accordingly, the thermal energy produced by a given element may be increased by increasing the current, or increasing the resistance of the resistance element, or a combination of both. The current through the heating element, however, is related to the resistance of the element by the formula: EQU I=E/R;
where E is the potential of the power source. Hence, the current through the element can also be increased by increasing the potential of the power source. This technique for increasing the heat liberated from a given area of heating element is generally not practical due to the cost of increasing the potential of the power source, and the thermal strain placed upon the heating element. There is a limit of how many watts per square inch of wire surface area can be liberated. If the watt input is too high the heating wire will burn out.
At temperatures above about 700.degree. C., practically all of the heat transferred from a heating element is through radiation, and it is only at lower temperatures that convection contributes to heat transfer. Since the combination thermal insulating and heating units of the present invention are intended for furnaces operating at temperatures well above 700.degree. C., only radiation can be considered for heat transfer from the element. Radiant heat transfer from the element, however, is a function of the temperature of the element and the surface area of the element, and can not be increased by external effects, as can be done with convection. Radiant heat transfer per unit of area of the heating element is a function of the temperature of the element, and there is a maximum operating temperature for a given heating element without accelerating deterioration of the element. Hence, increasing the radiant heat transfer from a given heating element by increasing the current through the element can only be practiced if the element is not operating at capacity.
The most practical technique for increasing the heat transfer from a given area of a heating unit operating above 700.degree. C. is to increase both the electrical current and the area of the heating element. The current through the electrical heating element can be increased by decreasing the resistance of the element or increasing the potential of the power source, but the area of the electrical resistance heating element of the inventor's U.S. Pat. No. 4,575,619 may not readily be increased. These electrical resistance elements require a plurality of 180 degree bends in opposite directions, and accordingly the elements must be of a material and size to permit formation. Further, the 180 degree bends are preferably on as short a radius as possible in order to maximize the length of the heating element disposed on a unit of area of the ceramic pad. In addition, the elements are constructed of solid resistance wire material to maximize the surface area for a given resistance per unit of length, and hence are subject to cracking during formation. As a result, it has proven to be difficult to increase the thermal output per unit of area of the combination thermal insulating and heating units constructed in the manner of U.S. Pat. No. 4,575,619.
It is therefore an object of the present invention to provide a combination thermal insulating and heating unit capable of operation at temperatures in excess of 700.degree. C. and constructed with a serpentine heating element in the manner of U.S. Pat. No. 4,575,619 which will transfer more heat per unit of area than the heating units of prior constructions without shortening the useful life of the heating element, and to provide a furnace utilizing the improved insulating and heating unit.
It is also an object of the present invention to provide such a combination thermal insulating and heating unit with the improved construction at a relatively small increase in cost over the prior construction.